The oxidation of sulfide ores by bacteria has been known for a number of years. The first realization that a certain type of bacteria is capable of oxidizing sulfide sulfur in combination with metals was advanced by Colmer and Hinkle in 1947. The name "thiobacillus ferrooxidans" was assigned to this bacteria around 1951.
In a general review article entitled "Use of Micro-Organisms for the Recovery of Metals" by O. H. Tuovinen and B. P. Kelly (1974), it is alleged that there are no insurmountable problems preventing scaling-up of laboratory scale fermenter systems to the multiple-thousand gallon level. Tuovinen et al. point out that: "Problems in scaling-up to be met are (a) adequate oxygen supply; (b) adequate carbon dioxide supply required for bacterial growth; (c) adequate agitation by mechanical stirring or air lift devices (stirring at high speeds insures rapid, efficient gas transfer in solution with minimum air-bubble size and the thorough mixing of the culture so that it and its effluent remain continuously homogenous); (d) the maintenance of low pH." However, in spite of the recognition of the need for adequate aeration and agitation, a commercial scale solution has not been previously disclosed.
A number of patents have recognized the importance of aeration. For example, U.S. Pat. No. 2,829,964 by Zimmerley et al. issued Apr. 8, 1958, discloses a process for regenerating ferric sulfate from ferrous sulfate solution. The ferric sulfate solution is employed as a lixiviant for the leaching of metal values from ores. Zimmerley et al. recognize the importance of aeration, however, only known methods for accomplishing it are discussed. Zimmerley et al. disclose that the regeneration is "ordinarily accomplished by mere aeration applied sufficiently vigorously to accomplish the desired regenerative result." It is also disclosed that aeration techniques of various kinds are well known and therefore do not constitute a part of the Zimmerley et al. invention. Consequently, Zimmerley et al. do not describe any new aeration process or apparatus. As examples of known methods for aerating the mixture, the following are disclosed as ways in which oxygen and carbon dioxide required by the bacteria during the conversion process can be supplied: (1) bubbling of compressed air through the solution; (2) vigorous agitation of the body of solution by mechanical means; (3) cascading of the solution; and (4) the provision of extensive surface area relative to the depth.
U.S. Pat. No. 3,305,353 by Duncan et al. issued Feb. 21, 1967, discloses a leaching process which comprises contacting sulfuritic ore with thiobacillus ferrooxidans while simultaneously agitating and/or aerating the mixture in order to oxidize ferrous iron, if present, and sulfide in the ore. It is disclosed that increased rates of bacterial activity are achieved due to an increased supply of oxygen and an improved opportunity for the bacterial cells to contact mineral surfaces. However, the aeration equipment employed by Duncan et al. was of the type which was commercially available at the time, for example as used in activated sludge sewage treatment plants. It is also disclosed by Duncan et al. that the metal is directly leached and placed in solution as a result of the microbiological process.
Two common techniques which are presently employed to aerate microbiological systems are: (1) high shear agitator systems; and (2) diffuser membrane systems. The high shear agitator systems typically employ an impeller and a means for introducing a gas near the impeller. The systems produce relatively small gas bubbles. However, the required energy input to rotate the impellers is relatively high. Additionally, the high shear generated by the impellers may have a negative effect on microorganisms. Membrane systems are also employed for forming small bubbles. However, the rate at which gas can be added to a system is limited by the transfer rate of the gas through the membrane. Also, because the membranes are typically located at the bottom of fermentation vessels, the gas must be supplied under high pressure if the vessel is deep.
Therefore, it would be advantageous if a method for aerating a commercial scale fermentation vessel in an efficient and economical manner could be provided. It would be advantageous if the method could be employed in the oxidation of an ore contained in a slurry. It would be advantageous if the method could improve the economical recovery of metal values from sulfide ores. It would be advantageous if the oxidation could be employed in conjunction with a cyanide leach process to economically and efficiently recover metal values on a commercial scale. It would be advantageous if the by-products from the oxidation process could be employed downstream to remove cyanide contained in tailings from a cyanide leaching process.